The M4 motorway was closed westbound between junctions 1 and 3 after a bus caught fire on Thursday night, the Highways Agency said.
The vehicle appeared to be completely engulfed in flames on a picture posted on Twitter, but there are no reports of any injuries.
London Fire Brigade said it received a number of calls from people reporting the blaze at 6.20pm, and four fire engines and 20 firefighters were sent to tackle the flames.
The Highways Agency said: "The M4 westbound has been closed between junctions 1 and 3 due to a vehicle fire.
"Road users on the A4 westbound heading out of London will not be able to join the M4 at junction 1 or 2 and may wish
instead to consider continuing their journey on the A4, to re-join the M4 at junction 3.
"There are long delays in this area."
A spokesman for the Highways Agency said road users should expect the A4 to be busier than usual due to the closure of the M4.